The present invention relates to an antiskid brake control system (or a wheel slip brake control system) for a vehicle, and more specifically to an antiskid brake control system arranged to improve the control performances by the application of a fuzzy control.
FIG. 24 shows a conventional antiskid brake system (Japanese patent post-examination publication No. 51-6305). The antiskid brake system of this conventional example includes a vehicle body acceleration sensor 1, a front wheel speed sensor 2, a rear wheel speed sensor 3, a front wheel control unit 4, and a rear wheel control unit 5. Each of the control units 4 and 5 determines a manipulated quantity, such as a brake fluid pressure, in accordance with the sensed vehicle body acceleration DV, the sensed wheel speed Vw of each wheel, and other control parameters, such as vehicle body speed V, and wheel acceleration DVw, obtained by calculation from DV and Vw. Each of the control unit 4 and 5 controls the braking force by sending a control signal representing the manipulated quantity to a wheel cylinder 6 or 7.
As shown by solid lines in FIG. 25, a coefficient .mu. of friction between a road surface and a wheel of a moving vehicle varies in dependence on a slip rate .lambda.. The slip rate .lambda. is expressed by a relationship between the vehicle speed V and the wheel speed Vw [.lambda.=(V-Vw)/V]. The friction coefficient becomes greatest in a zone around .lambda.=0.2. The slip rate at which the friction coefficient becomes greatest is approximately constant without distinction between a high friction road such as a dry paved road and a low friction road such as snow-covered road. Therefore, it is possible to always hold the friction coefficient near the maximum and to reduce the stopping distance by controlling the braking force so that the slip rate always remains approximately equal to 0.2. On the other hand, the cornering force of a wheel of a turning vehicle becomes smaller with increase of the slip rate, as shown by broken lines in FIG. 25, so that the directional control of the vehicle becomes more difficult with increase of the slip rate. Therefore, it is desirable to hold the slip rate equal to about 0.2 in order to brake and stop a vehicle safely and effectively.
Such a target is achieved in the conventional antiskid brake system as follows; Each of the control units 4 and 5 has a table TBL including values u.sub.1 -u.sub.n of the manipulated quantity, and determines the manipulated quantity by obtaining one of the values, corresponding to the wheel acceleration DVw and the slip rate .lambda.. When the brake is applied heavily, and the wheel speed Vw decreases rapidly, then the wheel acceleration DVw is increased in the minus direction, the slip rate is increased, and the tendency of the wheel to skid is increased. Therefore, the control unit 4 or 5 decreases the brake fluid pressure by choosing a low value (u.sub.8, for example) and sending a control signal representing the selected value to the wheel cylinder 6 or 7. Therefore, the wheel speed Vw is increased again, and the slip rate is restored to the zone around 0.2. When the wheel is accelerated by decrease of the brake fluid pressure, and the wheel acceleration DVw exceeds a predetermined value, then the control unit increases the brake oil pressure again in accordance with the slip rate. Thus, the antiskid brake system holds the average slip rate approximately equal to 0.2 by increasing and decreasing the brake actuating force repeatedly.
However, the conventional antiskid brake system is arranged to determined the brake oil pressure by using the table TBL in which the entire range of each of the wheel acceleration DVw and the slip rate is divided into a plurality of divisions. Therefore, the brake fluid pressure is changed in a stepwise manner, and accordingly the accuracy of control is not sufficiently high. It is possible to improve the accuracy by making the divisions of the wheel acceleration and the slip rate smaller, and increasing the number of the divisions. However, the accuracy of the conventional control system is limited by the accuracy in measurement of the slip rate. The slip rate is calculated by using the vehicle speed V relative to the ground. However, it is difficult to directly sense the ground speed. Usually, the ground speed is estimated from the difference between the front wheel speed and the rear wheel speed, or the vehicle acceleration DV, and the accuracy of the lip rate is not so high.